This invention generally relates to toolholders, and is particularly concerned with a toolholder assembly for accurately securing the shank of a punch in a compacting press of the type used to make cutting inserts.
Toolholders of the type utilizing set screws are well known in the prior art. Such toolholders generally comprise a collar having an annular wall. The inner surface of the annular wall defines a socket for receiving the end of a tool shank, while the outer surface of the wall defines the periphery of the collar. A set screw radially-oriented in the annular wall of the collar has a distal end that is extendible into and retractable from the socket recess. To secure a tool in the toolholder, one merely turns the set screw after the end of the tool shank has been inserted into the socket recess so that the tool shank is clampingly engaged between the end of the set screw and the opposing wall of the recess.
While set screw-type toolholders provide a simple and inexpensive mechanism for positively securing the end of a tool shank, the inventor has observed that such a mechanism is limited in the degree to which it can maintain an accurate alignment of the tool shank in the socket recess. Accurate alignment is necessary in many toolholder applications. One specific application involves the set screw type toolholder used to secure punches in compacting presses. These compacting presses may be used to manufacture cutting inserts. An example of such a compacting press 1 is illustrated in FIG. 1. Such presses include an upper plate assembly 3 having a set screw-type collar 5 for holding an upper punch 6. The punch 6 includes an upper ram 7 on the end of a shank 8 which may be cylindrical. The upper plate assembly 3 is reciprocally mounted with respect to a die table 13 by means of two guide rods 9a-b. The rods 9a-b are secured onto the sides of the upper plate assembly 3 by means of mounting screws 10a-b. Bottom portions of the guide rods are slidably mounted in cylindrical bushings 11a-b present in the sides of the die table 13. A forming cavity 15 is centrally disposed in the die table 13 just under the ram 7 of the upper punch 6. A stationary lower punch 17 with a lower ram 18 is located along the center line of the forming cavity 15, and extends no higher than the level of the die table 13. The lower punch 17 includes a lower ram 18 on the end of the punch. An annular die 19 circumscribes the forming cavity 15.
During the operation of the compacting press, carbide powder is loaded into the forming cavity 15, and the upper punch is forcefully pressed downwardly via a suitable hydraulic linkage (not shown) so that the carbide powder is compressed between the face of the upper ram 7, the lower ram 18 and the inner walls of the die 19. To insure the formation of a proper "green" insert (which is later further hardened by sintering), the dimensional tolerances between the outer diameter of the upper ram 7 and the inner diameter of the die 19 must be precise. Hence it is essential that the collar 5 maintain an accurate, center line alignment between the upper punch 6 and the forming cavity 15 defined by the die 19. In the past, set screw type collars have been used for this purpose. However, the applicant has observed that the deformation the set screw of the toolholder 5 causes to the collar can interfere with the accurate alignment between the upper punch and the forming cavity.
With specific reference now to FIGS. 2 and 3, such prior art toolholders 5 include a collar 23 circumscribed at its upper portion by an annular flange 25. Flange 25 facilitates the mounting of the toolholder 5 to the upper plate assembly 3 by means of bolts 27a,b (shown in FIG. 1). The collar 23 is formed in large part by an annular wall 29 whose inner surface defines a socket recess 31 for receiving the shank 8 of the upper punch 6, and an outer surface that defines the periphery 33 of the collar 23. The annular wall 29 has a radially-oriented threaded bore 35 for receiving a set screw 37. The distal end of the set screw 37 is received in an annular whistle notch 39 that circumscribes the proximal end of the punch shank 8. To minimize unwanted movement of the shank 8, the inner diameter of the socket recess 31 and the outer diameter of the shank 8 are dimensioned to very near the same size.
When the shank 8 is inserted into the recess 31 with the set screw 37 in a loosened condition, the shank 8 seats concentrically within the recess 31 as shown in FIG. 3. However, when the set screw 37 is tightened to insure that the shank 8 will not fall from the recess 31, the screw 37 applies a radial force to the annular wall 29 of the collar 23 which in turn results in an unwanted, lenticularly-shaped gap 40 as shown in FIG. 4. The gap 40 allows the shank 8 to rock about the set screw, thereby compromising the desired precise alignment between the axes of the cylindrical shank 8, and the annularly-shaped recess 31. This misalignment not only results in the manufacture of cutting inserts that are not within tolerances (which in turn necessitates a further grinding step or a complete discarding of the insert); it further can cause chipping of the edges of the upper and lower rams 7 and 18 and of the die 19, which in turn necessitates their replacement.
Clearly, there is a need for a toolholder capable of accurately mounting a punch or other tool in a desired alignment. While chuck and collet type toolholders are known which are capable of accurately aligning cutting tools, such toolholders require the precise machining of a relatively large number of parts and surfaces before such accurate alignment characteristics can be achieved. Accordingly, it would be desirable if such a toolholder were capable of achieving the same accurate tool alignment as collet and chuck-like toolholders by means of a mechanism that was far simpler in structure, and hence much easier and less expensive to manufacture.